Interactions of Rocks with Supercritical CO2 and Water in Enhanced Geothermal Systems and Beyond

There is an abundance of heat but lack of fluids underground. Current drilling and fracturing technologies can provide necessary access to the hot rocks, creating so-called Enhanced Geothermal Systems (EGS). Despite the 100-fold increase in the geothermal energy potential that EGS could yield, the reservoir development costs are very high, and bringing more water in sufficient quantity is not always economically viable. As an alternative fluid, supercritical CO2 has been considered for its potential to yield up to 50% more electricity than water, mainly due its lower viscosity and greater ability to permeate the reservoir. When only CO2 is present, the low dissolution strength will also help maintain the integrity of the rock formation. Nevertheless, there will always be some water and other reactive species in equilibrium with the rocks, and there are substantial risks associated with chain reactions triggered by introduction of a new fluid like CO2.